1. Field of Invention
The present invention relates to a method for isolating integrated circuit element. More particularly, the present invention relates to the method for manufacturing shallow trench isolation (STI).
2. Description of Related Art
In general, a complete integrated circuit is made from millions of MOS transistors. To prevent the short-circuiting of adjacent MOS transistors, an isolating dielectric layer referred to as "field oxide layer" (FOX) must be placed between two neighboring transistors. Alternatively, shallow trench isolation must be made by etching a trench between the neighboring transistors followed by filling in insulating material to define an active area.
FIGS. 1A through 1D are cross-sectional views showing the progression of manufacturing steps in the production of a conventional shallow trench isolation. First, as shown in FIG. 1A, a substrate 10 is provided. Then, a pad oxide layer 22 and a silicon nitride layer (Si.sub.3 N.sub.4) 24 are sequentially formed over the substrate 10. Conventional photolithographic and etching techniques are then used to define a trench 30 in the substrate 10.
Next, as shown in FIG. 1B, an oxide layer is deposited into the trench filling the trench and covering the silicon nitride layer 24. Later, a chemical-mechanical polishing operation is performed to planarize and remove the extra oxide material above the silicon nitride layer 24 forming an oxide layer 34.
Next, as shown in FIG. 1C, the silicon nitride layer 24 and the pad oxide layer 22 are removed to form the device isolation structure. The pad oxide is removed using a wet etching method with hydrofluoric acid solution as the etchant. Using such isotropic etching method, surface of the oxide layer 34 adjacent to the substrate 10 can be over-etched quite easily due to the immersion in hydrofluoric acid solution during the etching operation. This will result in the formation of recesses 38 on the surface of the oxide layer 34 adjacent to the substrate 10. Moreover, to protect the substrate surface, normally a sacrificial layer will be formed above the substrate after the formation of the field oxide regions. Hence, when the sacrificial layer is removed in a subsequent process, the hydrofluoric acid used in the removal process will also lead to an over-etching of the oxide layer 34 adjacent to the substrate 10.
Next, as shown in FIG. 1D, the recesses 38 formed on the surface of the oxide layer 34 adjacent to the substrate 10 exposes the substrate surface 10 only a little. Therefore, a thin gate oxide layer 40 will be grown there in a subsequent process. A thin gate oxide layer 40 not only will lower the reliability of the gate, but will also lead to the accumulation of charges near the upper trench comers, and hence the electric field surrounding that area is increased. This has the adverse effect of lowering the threshold voltage of the device and the generation of abnormal subthreshold current commonly referred to as a "kink" effect. Moreover, near the corner of the main active area, a corner parasitic MOSFET will be created, thereby leading to the development of a leakage current. The lowering of the threshold voltage, abnormal subthreshold current and current leakage all contribute to the lowering of device quality and the reduction of the product yield.
In light of the foregoing, there is a need to provide an improved structure and method of forming shallow trench isolation.